Limit Switch

Limit switch is a common small-current switch. It uses the collision of the mechanical moving parts to make their contacts make or break the control circuit, so as to achieve the purpose of mechanical control and limit protection. Usually, limit switches are used to limit the position or stroke of mechanical movement, so that the moving machinery can automatically stop, reverse movement, variable speed movement or automatic round-trip movement according to a certain position or stroke. The limit switch changes the operating state of the motor by converting the mechanical displacement into electrical signal. Therefore, limit switches are more widely used in the field of industrial automation. It can be equipped with different mechanical equipment to form more complex automation control system.

Difference between Limit Switch and Proximity Sensor

1. Proximity sensors are non-contact switches with a long life and are suitable for passing objects. Limit switches are contact switches. Due to the internal contacts, the working life is relatively shorter.
2. Limit switches are used for position detection in one direction. For two-way position detection, using a proximity sensor is better.
3. Use limit switch for protection function. Use a proximity sensor to detect when an object passes a certain point.
4. The limit switch usually has 1 NO + 1 NC contacts. Generally only one group of contacts is used, so there are 2 wires. Proximity sensors generally adopt 2-wire, 3-wire and 4-wire, so they have more wires than limit switches. Some proximity sensors have to confirm it is PNP or NPN based on the connected equipment.

How to Select a Limit Switch?

1. Select the limit switches according to the application scenarios and control object.
   When the movement speed of machine is not fast, the limit switches with the general application are usually used. When the path through which the machine travels is not appropriate for the installation of the roller plunger limit switches, the roller lever limit switches should be used.
   Besides, when there are very high requirements on the working efficiency, reliability and accuracy, another kind of switch - proximity sensor should be selected.
2. Select the appropriate actuator form according to the transmission force between the machine and limit switch, as well as the corresponding displacement relationship, that is, select the structural form of the limit switch according to the movement characteristics of the machine.
   Shell: According to the load of the limit switch, for light loads and medium loads, the shell material is mostly plastic. For heavy loads and super heavy loads, the shell material is mostly metal. Therefore, it is required to choose the appropriate shell.
   Actuator: Different acturatos are selected according to the shape and movement direction of the detection target. The types of actuators include roller plunger, top plunger, roller lever, adjustable roller lever, adjustable rod lever, fork lever, and coil spring.
3. Select the limit switches series according to the rated voltage and rated current of the control circuit.
4. According to the installation environment to choose the protection mode: open type or protection type.
5. According to the environmental factors such as ambient temperature, vibration and shock resistance, and protection class to select a appropriate limit switch. For example, the waterproof limit switch can be used in the industrial occasions required for waterproof, oil-resistant and dustproof.

What are the Application of Limit Switch?

1. The speed of the washing machine is very high in the process of dehydration (drying). If someone inadvertently opens the door or cover of the washing machine and then reaches in, it is easy to cause harm to people. In order to avoid this kind of accident, an electric contact is installed on the door or cover of the washing machine. Once someone opens the door or cover of the washing machine, it will automatically cut off the power of the motor, and even make the door or cover open by mechanical linkage, and immediately "brake" to force the rotating parts to stop, so as to avoid personal injury. 
2. In tape recorders and video recorders, we often use fast forward or rewind. The tape rotates rapidly, but it stops automatically when it reaches the end of the tape. Here, the travel switch is working again, but this time it is not caused by collision but by the sudden increase of the tension of the tape. The limit switch is mainly used to change the mechanical into electrical signal, so that the running state of the motor can be changed, so as to control the mechanical action or as program control.
3. The real use of limit switch is in industry, where it cooperates with other equipment to form more complex automation equipment. There are many such limit switches on the machine tool, which can control the movement of work piece or the stroke of automatic feed to avoid collision accidents Sometimes limit switch is used to make the controlled object automatically change direction between two specified positions, so as to obtain continuous reciprocating motion. For example, when the automatic material transportation car reaches the terminal, it touches the limit switch and turns on the dumper mechanism, then it overturns the materials in the car and returns to the starting point. After arriving at the starting point, it touches the travel switch of the starting point to connect the circuit of the charging mechanism and start the automatic loading. If it goes on like this, it becomes a set of automatic production line, which needs no human management and works day and night, saving people's physical labor.